An Insulated Gate Bipolar Transistor (hereinafter abbreviated as IGBT) is a semiconductor switching element in which a current flowing between a collector electrode and an emitter electrode is controlled by a voltage applied to a gate electrode. As a controllable power widely ranging from tens of watts up to hundreds of thousands of watts, as well as a controllable switching frequency widely ranging from tens of hertz up to and over hundreds of kilohertz, IGBTS are widely used for ranging from low electric power equipments such as an air conditioner and a microwave oven for domestic use, to high electric power equipments such as an inverter used for a railroad and ironworks.
The IGBT is required to have a low loss for high efficiency of the electric power equipments described above, and as such required to reduce a loss such as a conduction loss and a switching loss. At the same time, in order to prevent problems such as an EMC noise, a malfunction and a dielectric breakdown of a motor, what is required is a capability to control dv/dt in accordance with the specifications of applications.
Therefore, Japanese Patent Application Publication No. JP2000-307116A discloses an IGBT having a structure in which a dispositional gap of trenches 117 is changed, as shown in FIG. 10. At an IGBT in FIG. 10, a floating-p layer 105 is provided in a wide distance portion between the trenches 117, without forming a p-type channel layer 106.
According to this structure, as a current flows through only a narrow distance portion between the trenches 117, an over current flowing through at the time of the short circuit can be suppressed, hence avalanche capability of an element can be improved. In addition, as a part of hole current flows into the p-type channel layer 106 via the floating-p layer 105, a hole density in the vicinity of trenches 117 is increased, hence an ON-voltage of the IGBT can be lowered. Further, a p-n junction formed with the floating-p layer 105 and an n−-type drift layer 104 relaxes the electric field applied to corner portions of the trenches 117, thereby enabling to maintain a breakdown voltage.
However, in the conventional IGBT structure shown in FIG. 10, there is often a case that controllability over the time rate of change of an output voltage dv/dt of the IGBT and diodes of arm pairs is reduced when turning on the IGBT. In FIG. 11, an example of calculated waveforms of a collector-emitter voltage is shown when turning on the IGBT. As shown in FIG. 11, there is a period that the dv/dt of the collector-emitter voltage remains unchanged even the gate resistance is changed, therefore being uncontrollable.
The reason for this is considered as follows. That is, when the IGBT is turned on, holes flow transiently into floating-p layer 105 in FIG. 10, a voltage of the floating-p layer 105 is increased. At this time, as a displacement current flows into a gate electrode 110 via a feedback capacity formed with a gate insulation film 109, raising the gate voltage, the time rate of change of a collector current di/dt, which is determined by the product of a mutual conductance “gm” of a MOSFET structure and the time rate of change of a gate-emitter voltage dvge/dt, increases and then a switching speed is accelerated.
Amount of holes flowing transiently into the floating-p layer 105 is primarily determined by an internal structure of a semiconductor and difficult to control with an external gate resistance. Therefore, it is impossible to control the accelerated di/dt with the external gate resistance, and as a result there arises a time as shown in FIG. 11 when the time rate of change of the collector voltage dv/dt cannot be controlled by the gate resistance.
In order to suppress the raising of the gate voltage caused by the floating-p layer 105, following techniques are disclosed.
In a technique disclosed in Japanese Patent Application Publication No. JP2004-039838A, by electrically connecting the floating-p layer 105 and the emitter electrode 114 via a resistor 301 as shown in FIG. 12, the raising of voltage of the floating-p layer 105 is suppressed. In this manner, a displacement current flowing into the gate electrode 110 from the floating-p layer 105 is reduced, hence raising the gate voltage is suppressed, thereby improving the controllability of the dv/dt as a result.
In a technique disclosed in Japanese Patent Application Publication No. JP2005-327806A, by filling in the wide region between the trenches with an insulation film, the floating-p layer is removed and by eliminating voltage variation at the gate caused by the floating-p layer, the controllability of the dv/dt can be improved. Further, as one side of the gate electrode is covered with a thick insulation film, the feedback capacity can be reduced, and thereby the controllability of the dv/dt can be more improved.
In a technique disclosed in Japanese Translation of PCT Patent Application Publication No. JP2002-528916A, by providing in a trench a gate electrode at the upper side and an embedded electrode, which is connected to the source electrode, at the lower side via an insulation film, the feedback capacity of the gate can be reduced.